1. Field of the Invention
The present invention relates to a method for fabricating a micro-electro-mechanical structure, and more particularly to a method for fabricating a sealed cavity microstructure, which can effectively avoid improper erosion, reduce the probability of exposure and further reduce the package cost by being packed and integrated with a general integrated circuit.
2. Description of the Prior Art
The existing semiconductor micro-electro-mechanical system include various semiconductor microstructures, such as the unmovable probe, channel, cavity structure, or the movable spring, linkage rod, gear (rigid body movement or flexible deformation), etc.
Integrating the above different structures with the related semiconductor circuit can form various semiconductor applications. Thus, how to utilize the fabricating method to improve the various functions of the microstructure is the key index of the semiconductor electromechanical system in the future and is also a rigorous challenge of further developing the chip in the future.
The existing method for fabricating the micro-electro-mechanical sensor and actuator system is often required to fabricate a suspended structure on a silicon substrate. The above process must adopt the advanced semiconductor technology such as dry etching, wet etching and gas etching to remove the sacrificial layer and form suspension micro-electro-mechanical systems inside components. These micro-electro-mechanical systems can cause high sensitive action and can be widely applied to the production designs.
A conventional method disclosed in U.S. Pat. No. 6,458,615 B1 is to form at least one insulation layer including an inner micro-electro-mechanical structure on an upper surface of a silicon substrate, and then conduct a layer-by-layer etching operation from the upper surface until the lateral edge of the micro-electro-mechanical structure, and finally, conduct an isotropic dry etching to the silicon substrate to achieve the suspension of the micro-electro-mechanical structure.
The above conventional method can be used to fabricate a suspended micro-electro-mechanical structure, but it has the following disadvantages:
First, it adopts anisotropic dry chemical etching and uses chemical reaction to remove the isolation layer, however, after the side edge of the micro-electro-mechanical structure is etched, the silicon substrate still needs to be massively etched by isotropic chemical etching, and this technique will produce serious undercut problems;
Second, in the process of this conventional technology, the micro-electro-mechanical structure is exposed in the process at first, after a long time of multi-layer processing, the exposed micro-electro-mechanical structure is likely to be contaminated and damaged, causing an excessively low yield rate;
Third, after the etching operation is completed, the micro-electro-mechanical structure has already been capable of operating in suspended state, but a special machine will be used to pack the micro-electro-mechanical structure surface to block dust and particle. However, since the micro-electro-mechanical structure must be ensured in the suspended state, the conventional method is to place a special mold used as a cap over the product surface, and then precisely fabricate a package protection cap without touching the suspended micro-electro-mechanical structure. This kind of surface package is complicated and expensive, and unlikely to be integrated with the process of the common integrated circuit package.
With the rapid development of the above technology, in order to solve many problems, U.S. Pat. No. 7,008,812B1 discloses a manufacture of MEMS structures in sealed cavity which utilizes two sacrificial layers to clad the suspended micro-electro-mechanical part from the upper side and the lower side, respectively. Moreover, after the sacrificial layer utilizes a cap structure to protect the inner suspended micro-electro-mechanical part, holes are made in the cap structure to etch the two sacrificial layers inside the cap structure. Outside the cap structure is covered another protective layer to define a sealed cavity, thus realizing the effective suspension of the micro-electro-mechanical suspended part. This technology can greatly reduce the probability of the micro-electro-mechanical exposure, but it has the following disadvantages:
The two sacrificial layers must clad the suspended micro-electro-mechanical part from the upper side and the lower side respectively, the fabrication of the two sacrificial layers wastes more time and labour, and the mask alignment and photolithography technology used together with the micro-electro-mechanical suspended part is quite fine, so this improved technology is still more troublesome and complicated;
The above problems caused in the sealed cavity micro-electro-mechanical structure fabrication haven't been solved yet, and they are also the key points of continuously looking for breakthroughs in the existing semiconductor microstructure design and fabrication, and the package industry.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.